Mapping of cavitating flow regimes in injectors for medium-/heavy-duty diesel engines

Reducing the sac volume size of medium-/heavy-duty diesel engine injector nozzles can minimise the fuel dripping into the combustion chamber at the end of injection events, which has been linked to reduced engine-out emissions. This study demonstrates the effect of reduction in the sac volume of diesel fuel injectors utilised in medium-/heavy-duty applications on the internal nozzle flow. This is realised by comparison of two heavy-duty diesel nozzles that feature a large difference in sac volume size of almost three times. For visualisation purposes, the nozzles have been enlarged by six times, and replicas were manufactured from a transparent material. High-speed digital imaging was used to capture the instantaneous spatial and temporal characteristics of geometric as well as dynamic vortex cavitation structures. The investigation was conducted in a steady-state flow test rig for three different needle valve lifts. For all tested conditions, the flow behaviour was analysed at three distinct areas of the nozzle, these being the needle seat, the sac volume and the injection hole. Interpretation of experimental observations was supported by parallel computational fluid dynamics simulations of the exact conditions measured during the experiments. Post-processing of the captured images has revealed the ensemble – average cavitation location, its standard deviation and the cavitation structures life – time inside the sac volume. Results showed a significant dependency of the internal nozzle flow on the sac volume size and identified clear differences in the structure of the cavitation pockets inside the sac volume under certain operating conditions

Cavitation Inside Enlarged And Real-Size Fully Transparent Injector Nozzles And Its Effect On Near Nozzle Spray Formation

The effect of string cavitation in various transparent Diesel injector nozzles on near nozzle spray dispersion angle is examined. Additional PDA measurements on spray characteristics produced from real-size transparent nozzle tips are presented. Highspeed imaging has provided qualitative information on the existence of geometric and string cavitation, simultaneously with the temporal variation of the spray angle. Additional use of commercial and in-house developed CFD models has provided complimentary information on the local flow field. Results show that there is strong connection between string cavitation structures and spray instabilities. Moreover, elimination of string cavitation results in a stable spray shape that is only controlled by the extent of geometric-induced cavitation pockets. Finally, PDA measurements on real-size transparent nozzle tips have confirmed that such nozzles reproduce successfully the sprays generated by production metal nozzles

Experimental Study of Diesel-Fuel Droplet Impact on a Similarly Sized Polished Spherical Heated Solid Particle

The head-to-head impact of diesel-fuel droplets on a polished spherical brass target has been investigated experimentally. High-speed imaging was employed to visualize the impact process for wall surface temperatures and Weber and Reynolds numbers in the ranges of 140–340 °C, 30–850, and 210–1135, respectively. The thermohydrodynamic outcome regimes occurring for the aforementioned ranges of parameters were mapped on a We–T diagram. Seven clearly distinguishable postimpact outcome regimes were identified, which are conventionally called the coating, splash, rebound, breakup–rebound, splash–breakup–coating, breakup–coating, and splash–breakup–rebound regimes. In addition, the effects of the Weber number and surface temperature on the wettability dynamics were examined; the temporal variations of the dynamic contact angle, dimensionless spreading diameter, and liquid film thickness forming on the solid particle were measured and are reported

Spray characteristics of a multi-hole injector for direct-injection gasoline engines

The sprays from a high-pressure multi-hole nozzle injected into a constant-volume chamber have been visualized and quantified in terms of droplet velocity and diameter with a two-component phase Doppler anemometry (PDA) system at injection pressures up to 200 bar and chamber pressures varying from atmospheric to 12 bar. The flow characteristics within the injection system were quantified by means of a fuel injection equipment (FIE) one-dimensional model, providing the injection rate and the injection velocity in the presence of hole cavitation, by an in-house three-dimensional computational fluid dynamics (CFD) model providing the detailed flow distribution for various combinations of nozzle hole configurations, and by a fuel atomization model giving estimates of the droplet size very near to the nozzle exit. The overall spray angle relative to the axis of the injector was found to be almost independent of injection and chamber pressure, a significant advantage relative to swirl pressure atomizers. Temporal droplet velocities were found to increase sharply at the start of injection and then to remain unchanged during the main part of injection, before decreasing rapidly towards the end of injection. The spatial droplet velocity profiles were jet-like at all axial locations, with the local velocity maximum found at the centre of the jet. Within the measured range, the effect of injection pressure on droplet size was rather small while the increase in chamber pressure from atmospheric to 12 bar resulted in much smaller droplet velocities, by up to four-fold, and larger droplet sizes by up to 40 per cent

Multihole injectors for direct-injection gasoline engines

High-pressure multi-hole nozzles, carrying a Diesel-derived technology, are believed to be promising Fuel Injection Equipment (FIE) for Direct-Injection (DI) Spark Ignition (SI) gasoline engines. Having explored thoroughly swirl pressure atomisers and their spray behaviour, multi-hole nozzles represent the second-generation injectors. Thus, complete investigation of multi-hole nozzle flow, spray characteristics and their engine performance is a vital part of development of future DI gasoline engines. The internal nozzle flow of an enlarged transparent multi-hole injector was investigated for different flow rates and needle lifts under steady state flow conditions. High-resolution CCD camera and high speed digital video systems were employed to visualize the nozzle flow patterns and cavitation development. The images identified the onset of cavitation in multi-hole gasoline nozzles and revealed the transition from pre-film to film stage cavitation. Cavitation strings were also visualized inside the injection hole that could extend to the needle face. However, these structures are highly unstable and directly affected by needle lift and cavitation number, although it appeared to be independent of the Re, in a behaviour similar to that of multi-hole diesel injectors. The sprays from various high-pressure multi-hole nozzle designs injected into a high-pressure/temperature constant-volume chamber have been visualised and quantified in terms of droplet velocity and diameter with a two-component phase-Doppler Anemometry (PDA) system at injection pressures up to 200bar and chamber pressures varying from atmospheric to 12bar. The overall spray angles relative to the axis of the injector were found to be almost independent of injection and chamber pressure, a significant advantage relative to swirl pressure atomisers. Within the measured range, the effect of injection pressure on droplet size was rather small while the increase in chamber pressure from atmospheric to 12bar resulted in much smaller droplet velocities, by up to fourfold, and larger droplet sizes by up to 40%. The effect of chamber temperature on multi-hole sprays confirmed the expected trends that dictate smaller droplet size distributions as temperature rise from 50 to 90 and 120°C. Additionally, multiple-injection proved to have similar dependencies to the single injection with certain operating limits. Laser-induced fluorescence has been mainly used to characterise the two-dimensional fuel vapour concentration inside the cylinder of a multi-valve twin-spark ignition engine equipped with high-pressure multi-hole injectors. The effects of injection timing, in-cylinder charge motion and injector tip layout have been quantified. The flexibility in nozzle design of the multi-hole injectors has proven to be a powerful tool in terms of matching overall spray cone angle and number of holes to specific engine configurations. Injection timing was found to control spray impingement on the piston and cylinder wall, thus contributing to quick and efficient fuel evaporation. Multipleinjection performed well under certain operating conditions and proved to be a powerful tool in the hands of engine manufacturers. It was confirmed that in-cylinder charge motion plays a major role in engine's stable operation by assisting in the transportation of the air-fuel mixture towards the ignition locations (i.e. spark-plugs) in the way of a uniformly distributed charge or by preserving stratification of the charge depending on operating mode of the engine

Internal flow and cavitation in a multi-hole injector for gasoline direct-injection engines

A transparent enlarged model of a six-hole injector used in the development of emerging gasoline direct-injection engines was manufactured with full optical access. The working fluid was water circulating through the injector nozzle under steady-state flow conditions at different flow rates, pressures and needle positions. Simultaneous matching of the Reynolds and cavitation numbers has allowed direct comparison between the cavitation regimes present in real-size and enlarged nozzles. The experimental results from the model injector, as part of a research programme into second-generation direct-injection spark-ignition engines, are presented and discussed. The main objective of this investigation was to characterise the cavitation process in the sac volume and nozzle holes under different operating conditions. This has been achieved by visualizing the nozzle cavitation structures in two planes simultaneously using two synchronised high-speed cameras. Imaging of the flow inside the injector nozzle identified the formation of three different types of cavitation as a function of the cavitation number, CN. The first is needle cavitation, formed randomly at low CN (0.5-0.7) in the vicinity of the needle, which penetrates into the opposite hole when it is fully developed. The second is the well known geometric cavitation originating at the entrance of the nozzle hole due to the local pressure drop induced by the nozzle inlet hole geometry with its onset at around CN=0.75. Finally, and at the same time as the onset of geometric cavitation, string type cavitation can be formed inside the nozzle sac and hole volume having a strong swirl component as a result of the large vortical flow structures present there; these become stronger with increasing CN. Its link with geometric cavitation creates a very complex two-phase flow structure in the nozzle holes which seems to be responsible for hole-to-hole and cycle-to-cycle spray variations

A numerical simulation of single and two-phase flow in porous media: A pore sale observation of effective microscopic forces

Modelling fluid flow in rock porous media is a challenging physical problem. Simplified macroscopic flow models, such as the well-known Darcy's law, fail to predict accurately the pressure drop because many flow parameters are not considered while simplifications are made for the multi-scale structure of the rocks. In order to improve the physical understanding for such flows and the accuracy of existent models, there is a need for realistic geometries to be investigated. The present work describes initially single-phase flow simulations performed on numerical grids obtained from reconstruction of 2D images of rock porous media found in the open literature using ANSA®. The results in terms of preferential paths and tortuosity are compared with experiments. Following, multiphase flow models have been utilised focusing on the capturing of the liquid-gas interface motion. It is concluded that for such complex porous rock problems, the multi-scale flow development is grid dependent

Friction-induced heating in nozzle hole micro-channels under extreme fuel pressurisation

Fuel pressurisation up to 3000 bar, as required by modern Diesel engines, can result in significant variation of the fuel physical properties relative to those at atmospheric pressure and room temperature conditions. The huge acceleration of the fuel as it is pushed through the nozzle hole orifices is known to induce cavitation, which is typically considered as an iso-thermal process. However, discharge of this pressurised liquid fuel through the micro-channel holes can result in severe wall velocity gradients which induce friction and thus heating of the liquid. Simulations assuming variable properties reveal two opposing processes strongly affecting the fuel injection quantity and its temperature. The first one is related to the de-pressurisation of the fuel; the strong pressure and density gradients at the central part of the injection hole induce fuel temperatures even lower than that of the inlet fuel temperature. On the other hand, the strong heating produced by wall friction increases significantly the fuel temperature; local values can exceed the liquid’s boiling point and even induce reverse heat transfer from the liquid to the nozzle’s metal body. Local values of the thermal conductivity and heat capacity affect the transfer of heat produced at the nozzle surface to the flowing liquid. That creates strong temperature gradients within the flowing liquid which cannot be ignored for accurate predictions of the flow through such nozzles

Cloud cavitation vortex shedding inside an injector nozzle

The development and collapse of cloud cavitation and its link to surface erosion within a transparent test single-orifice nozzle operating with a closed Diesel fuel hydraulic circuit, has been characterized using high-speed imaging. Data have been obtained for a range of cavitation and Reynolds numbers under fixed lift positions. Post processing of a large number of images acquired with short exposure time (1 μs) allowed the elucidation of the distinct flow phenomena associated with the highly transient two-phase flow. At the inlet of the flow orifice, the vapour cloud was found to occupy the largest part of the nozzle hole cross-section. Coherent vortical structures of a hairpin shape have been detected to onset at the closure region of this vapour cloud and shed downstream in a fully transient manner. The effect of the operating parameters on the temporal and spatial characteristics with regards to the emergence and collapse of the hairpin vortices has been quantified. It has been established that the cavitation-vortex shedding was taking place in a periodical manner, characterized by a Strouhal number